The U.S. Pat. No. 7,701,211 discusses an MR imaging system in which desired RF excitation fields (also referred to as B1 fields) are obtained in a fast and easy way. This object is achieved by a method of operating an MR imaging system, the method comprising the steps of determining complex weighting factors for a plurality of RF coils by measuring the transmit sensitivities of said RF coils and calculating the weighting factors for a given RF excitation, generating a plurality of individual coil drive signals, and individually setting the amplitude and/or phase of each of said coil drive signals according to the respective weighting factor.